1. Technical Field
This invention relates generally to diesel engines and more particularly to coatings used to protect the various power cylinder components of the power cylinder including the piston.
2. Related Art
Many ways and means exist to protect in-cylinder diesel engine components from deleterious effects of combustion by-products, notably when EGR (exhaust gas recirculation) is used in the engine. Also, it has been proposed to use coatings as joint tribological elements in lieu of bushings and bearings and the like. As examples of the latter, manganese phosphate, DLC (diamond-like-coatings), many variations of PVD (physical vapor deposition)-derived coatings, electroless and electroplated layers.
With widespread use of high pressure diesel engine injection systems (common rail, electronic unit injectors, hydraulic electronic unit injectors) piston bowl rim oxidation/erosion by the torch-like effects from each combusting fuel plume are becoming commonplace. Also, due to late fuel injection timing necessary to preclude NOx formation, instances of liner fuel overspray have been documented. On the other extreme, early or pilot injection might deposit liquid fuel on piston tops that remains unburnt for most of the compression stroke. There are several side effects to these conditions inherent with high pressure diesel injections systems, including: 1) the build-up of carbon deposits on the top land of diesel engine pistons, 2) the deterioration of the oil film at top dead center/top ring reversal point on the liner and 3) piston erosion (pitting) particularly where the plumes from the injectors fire on the top bowl edge of the piston.
Top land carbon build up is historically singled out as the root cause of bore polishing and subsequent lubricant oil control loss. Emissions are then adversely affected.
Liner overspray with fuel dilutes and contaminates the oil film, greatly affecting the ability of the squeeze film to handle the first ring contact load. Fuel overspray also depletes the lubricant TBN reserve and entrains soot into the oil film. The net effect is the formation of a corrosive/abrasive medium which wears out components in intimate contact with the medium. Chemical etching (corrosion) of the liner on the first 20 to 40 mm of the piston travel from TDC not only leads to accelerated local wear, but also leads to higher ring peripheral (OD) face wear.
Local uncontrolled (explosive) deposited fuel burn creates extremely high thermal and mechanical stressing of the piston material. Eventually pitting occurs and will structurally weaken reentrant combustion bowl overhang lip and breakage can occur. Further out towards the piston OD, craters can be dug into the metal substrate. Emissions and overall engine performance are adversely affected. These factors may combine in such a way as to aggravate first ring groove wear in high pressure/high thermal loaded diesel engines.
What has been described thus far are typical problems, to a greater or lesser degree, derived from in-cylinder environment created by emissionized engines having EGR. In addition, the high rate of pressure rise and high peak combustion pressure characteristic of modern diesel engines slap the piston against and excite the liner's vibratory motions, triggering liner cavitation which can be a major problem itself. Skirt coatings (polymer based or graphite impregnated) are expected to help diminish cavitation but they bring additional problems of their own, besides being cumbersome to apply during manufacture.